In recent years, a polymerizable liquid crystal compound is used as a film having optical anisotropy. The compound has optical anisotropy in a liquid crystal state, and the alignment of the liquid crystal compound is immobilized through polymerization. The addition of a polymerizable optically active compound to a polymerizable liquid crystal compound induces a helical structure, and various applications as optical devices are available depending on the pitch of the helical structure. That is, the propagation of light along the helical axis is classified depending on the wavelength (λ) of the target light and the length of the helical pitch (P) into the case (1) where λ<<P and the case (2) where λ≈P.
In the case where the target λ is visible light, the case (1) where λ<<P corresponds to 1 (μm)<P. The application in this case is classified into the case where the Mauguin condition is satisfied and the case where it is not. In the case (A) where the Mauguin condition is satisfied, i.e., the condition where Φ<<2πΔnd/λ is satisfied, linear polarized light agreeing with or perpendicular to the optical axis on the incident side is emitted as linear polarized light maintained, whereby the device functions as a rotator. Herein, Φ represents the total twist angle, d represents the thickness, and Δn represents the birefringence of the liquid crystal. In the case (B) where the Mauguin condition is not satisfied, the linear polarized light shows birefringence that is determined by Φ, d and Δn. The rotator can be applied as an optical device for a head-up display and a projector. An application of birefringence of twist alignment includes, for example, optical compensation in an STN (super twisted nematic) type liquid crystal display (as described in JP-A No. 8-87008).
Where the target λ is visible light, in the case (2) where λ≈P, for example, where the twist direction of the helical structure is right hand, the liquid crystal film selectively reflects only clockwise circularly polarized light having a wavelength λ in a range of no×P<λ<ne×P (where no represents the refractive index of the liquid crystal layer to normal light, and ne represents the refractive index of the liquid crystal layer to abnormal light), and transmits all clockwise circularly polarized light having a wavelength outside the range and anticlockwise circularly polarized light having any wavelength. In other words, clockwise circularly polarized light and anticlockwise circularly polarized light can be selectively separated from each other at a specific wavelength (circularly polarized light dichroic property). From the standpoint of application of an optical device, specifically, it is classified into the case (A) where 350/nave (nm)<P≦800/nave (nm), i.e., the wavelength range of the circularly polarized light dichroic property is in the visible region, and the case (B) where P<350/nave (nm), i.e., the wavelength range of the circularly polarized light dichroic property is in the ultraviolet region (where nave=((ne2+no2)/2)0.5).
In the case (A) where 350/nave (nm)<P≦800/nave (nm), when unpolarized light is incident, reflected light and transmitted light are colored corresponding to the wavelength causing the circularly polarized light dichroic property. By using the coloration, the device can be applied to a color filter used for design purpose, such as ornament materials, and for a liquid crystal display device. The device can also be applied to anticounterfeit technology since reflected light and transmitted light have unique metallic luster with change in color tone depending on viewing angle, and these optical characteristics cannot be reproduced by a duplicator. Furthermore, the light utilizing efficiency in a liquid crystal display device can be improved by utilizing the circularly polarized light separation function. For example, such a constitution has been proposed that a ¼λ plate and an optically anisotropic film exhibiting the circularly polarized light separation function are accumulated on a polarizing plate (as described in Y. Hisatake, et al., Asia Display/IDW '01, LCT8-2). In these purposes, it is demanded to exhibit the circularly polarized light separation function over the entire visible light region (region having a wavelength of from 350 to 750 nm), layers having different pitches may be accumulated, or in alternative, the pitch may be changed consecutively in the thickness direction. The reflection spectrum width Δλ is larger when the birefringence anisotropy value (Δn) is large owing to the relational expression Δλ=Δn×P. The reflection spectrum center wavelength λc is calculated from the relational expression λc=nave×P. When the helical pitch P is set in a range of 700/nave (nm)<P≦1.5/nave (μm), such an application as a reflection filter for an ultraviolet ray or a near infrared ray can be attained by utilizing the similar circularly polarized light separation function.
In the case (B) where P≦350/nave (nm), the refractive index in the visible range on a plane perpendicular to the helical axis is expressed by ((ne2+no2)/2)0.5, and the refractive index in the visible range in the direction of the helical axis is equal to no (as described in W. H. de Jeu, Physical Properties of Liquid Crystalline Materials, Gordon and Breach, New York (1980)).
The optically anisotropic film having the optical characteristics is referred to as a negative C-plate. In a liquid crystal display device that shows black display (dark state) when liquid crystal molecules exhibiting positive birefringence are oriented in the direction perpendicular to the substrate, no birefringence is exhibited in the normal line direction of the display device by alignment of the liquid crystal molecules. In the display device, accordingly, a considerably high contrast can be obtained in the normal line direction. However, birefringence is exhibited in directions deviated from the normal line direction to increase the transmittance in the black display (dark state). In other words, the display device is decrease in contrast in the oblique viewing angle. The negative C-plate can compensate the birefringence caused in directions deviated from the normal line direction of the liquid crystal alignment direction in the display device. As a result, the negative C-plate can be used as an optical compensation plate suitable for improving the viewing angle characteristic in a display device as VA (vertically aligned), TN (twisted nematic), OCB (optically compensated birefringence) and HAN (hybrid aligned nematic).
A compressed polymer film or a film utilizing planarly oriented discotic liquid crystal having a negative birefringence is currently used as an optical compensation film (as described in JP-A No. 2002-6183). The use of a polymer of cholesteric liquid crystal formed of liquid crystal molecules having a positive birefringence enhances the degree of freedom upon designing the refractive index anisotropy value and the wavelength dispersion thereof. The negative C-plate can be used in combination with various optical compensation layers.
The pitch and Δn are appropriately controlled depending on the optical design for the aforementioned purposes.
In any of the aforementioned purposes, the photopolymerizable liquid crystal before curing is demanded to have such characteristics that it has a nematic phase at room temperature and a wide nematic phase, exhibits a good alignment property, and is rapidly cured through UV irradiation. Furthermore, such a photopolymerizable cholesteric liquid crystal composition is demanded that provides a cured product having suitable Δn and transparency, and being excellent in heat resistance and humidity resistance, as characteristics after curing.
Upon optimizing a compound, it is necessary to satisfy the polymerization property and the physical and chemical properties of the polymer, in addition to the aforementioned optical characteristics. The physical and chemical properties include the polymerization rate and the polymerization degree of the compound, and the transparency, the mechanical strength, the coating property, the solubility, the degree of crystallinity, the contraction property, the water permeation property, water adsorption property the gas permeation property, the melting point, the glass transition point, the clearing point, the heat resistance and the chemical resistance of the polymer.
The addition of an optically active compound to a liquid crystal composition induces a helical structure (as described in GB-A No. 2,298,202 and WO No. 02/28985). The pitch (p) depends on the addition amount (concentration c) and the helical twisting power (HTP) of the optically active compound (p=HTP−1×c−1). A liquid crystal composition having a helical structure can be applied to various purposes. Examples of the purposes include a PC (phase change) display device, a guest-host display device, a TN display device, an STN display device, a SSCT (surface stabilized cholesteric texture) display device, a PSCT (polymer stabilized cholesteric texture) display device and a negative C-plate.
In any of the purposes, it is preferred that the addition amount of the optically active compound is minimized to prevent the various properties, such as the viscosity and the liquid crystal property, from being adversely affected. Accordingly, an optically active compound having large HTP is demanded. Furthermore, an optically active compound is generally low in solubility in a liquid crystal composition to make difficult the addition amount thereof large, and therefore, an optically active compound having large HTP is demanded.
A molded article having optical anisotropy is utilized in such applications as an optical compensation plate, such as a polarizing plate a negative C-plate, an alignment film, a color filter, an adhesive, a synthetic polymer having mechanical anisotropy, a cosmetic product, an ornamental product and an anticounterfeit apparatus. Such a molded article is demanded that is excellent in polymerization degree, transparency, mechanical strength, coating property, solubility, degree of crystallinity, contraction property, water permeation property, water absorption property gas permeation property, melting point, glass transition point, clearing point, heat resistance and chemical resistance of the polymer. Furthermore, it is demanded that a liquid crystal compound containing an optically active compound is excellent in polymerization rate.